When attempting to reduce the thickness of silicon solar cells, a decreasing efficiency of the solar cell is observed. This is due, on the one hand, to the no longer complete absorption of sunlight when there is a thinner absorption length. On the other hand, charge carriers are increasingly generated in proximity to the back, with minority charge carriers being able to reach the back electrode as a result of diffusion, thereby reducing the current generated by the majority charge carriers.
A highly doped layer on the back makes it possible to generate a field that counteracts the diffusion of minority charge carriers, a so-called back-surface field. In the case of a solar cell structure having a p-doped solar cell body and an n+ doped emitter on the solar cell's light-incidence or front side, a p+ doping is necessary on the back for this purpose. Aluminum, which is applied as a thin layer for example by vapor deposition on the back and which can be driven in or alloyed by means of an annealing step, is very often suggested in order to generate this p+ doping. It is also possible to generate the p+ doping by applying back contacts that contain aluminum and by correspondingly driving in the aluminum. It is further possible to diffuse aluminum from a solid diffusion source into the solar cell substrate. This nevertheless suffers from the drawback that the solar cell substrate is doped with aluminum on both sides, thus generating a p+pp+ structure.
Boron is also suitable for p-doping generation. A corresponding back-surface field can be generated by gaseous diffusion of a correspondingly volatile or gaseous boron compound, by applying a silicon layer that contains boron on the back or by applying a liquid solution that contains a dopant. Due to the high volatility of the boron compounds, however, an all-over diffusion which has to be prevented by masking those solar cell regions that are not to be doped is constantly observed at the temperatures necessary for driving in the doping.
The p+ doping--which is simple to produce according to the process--with aluminum suffers from the disadvantage of an increased susceptibility to corrosion. Over time, layer regions that contain aluminum may decompose and peel off, which may result in damage to the back contacts and cause a reduction in solar cell performance.
A method of producing a silicon solar cell is known from IEEE PHOTOVOLTAIC SOLAR ENERGY CONFERENCE, Oct. 12-16, 1992, Montreux, Switzerland, pages 164-167; in this method, a boron-doped oxide layer is applied on the back of a silicon wafer and then the boron is diffused into the silicon at a temperature of 940.degree. C. An emitter layer is subsequently generated by means of phosphorus diffusion before contacts are applied by etching.
A method of producing a silicon solar cell which is particularly directed at the production of doped regions is known from PCT reference WO 91/19323. An oxide-forming mask layer containing a dopant is applied to part of the surface of a semiconductor substrate, and the substrate is then heated to a temperature sufficient for the diffusion of part of the dopant from the mask layer into the semiconductor layer, with the bare semiconductor substrate surface also being autodoped. The semiconductor substrate's autodoped regions are etched off, while the mask layer represents a protective layer for the doped regions beneath the mask layer.